Why is "battered woman syndrome" a defense for murder?

If a guy hits a woman, he should go to jail. She shouldn't kill him. End of story. If he's literally running after her with a knife THEN she can kill him. She can't just shoot him because he slapped her a few times, and she can't just kill him because he punched her two days ago. Call the fricking police, it's what they are there for. And better yet, get the hell away from him. Duh.